For general information on trade associations, look in the Encyclopedia of Associations, which is available as a multi-volume hard cover, on the Gale Directory Library and on Lexis (ENASSC). Also check to see if the association has a Web site, and check the relevant news sources for articles.
Reports: The Investext Group sells reports published by trade associations. If you are looking for a specific report, it may be cheaper to contact the association directly (or an industry-specific library, if you know of one). For older reports, you can also search OCLC's WorldCat.
Treatises: A good comprehensive treatise on this topics is The Law of Associations (LexisNexis) which is available as a one-volume print treatise and on Lexis (LAWASC).